The recent failure of vaccines based solely upon generating cytotoxic T lymphocyte (CTL)responses underscores the need to mobilize humoral as well as cellular immunity. Project 2 seeks to test the hypothesis that a potent CTL vaccine can be protective if additional coverage is provided by neutralizing antibodies (nAbs). We will use passive immunization with SHIVIG, i.e., polyclonal IgG isolated from rhesus macaques (RM) with high-titer, broadly reactive anti-HIV nAbs in response to infection with simian-human immunodeficiency virus (SHIV) strains encoding R5 env genes of recently transmitted HIV clade C (HIV-C). We seek to address the following Specific Aims: 1. Correlate in vitro neutralization in PBMC-based assays with in vivo protection against a Tier 1 R5 SHIV using passive immunization with SHIVIG. Three groups of RM will be given passive immunoprophylaxis with SHIVIG at different doses; the control group will receive normal RM IgG. All four groups will be challenged intrarectally (i.r.) with weekly, low-dose inoculations of R5 SHIV (Tier 1). 2. Correlate in vitro neutralization in PBMC-based assays with in vivo protection against a Tier 2 R5 SHIV-C using the experimental approach of Aim 1. 3. Combine passive SHIVIG immunization with active vaccination based upon the optimal CTL vaccine as determined by the mucosal immunogenicity studies of Dr. Silvestri (Project 3, Aim #1). The challenge will involve a Tier 1 SHIV strain with multiple low-dose i.r.challenges. 4. Induce nAb responses by active immunization with HIV-C Env immunogens in combination with the optimal CTL vaccine in a bimodal vaccine strategy. We are currently developing HIV-C Env immunogens based upon native trimeric, recently transmitted Env's and mutants thereof (with support from a different HIVRAD) and have seen the induction of nAb responses against heterologous primary R5 strains. We will employ the best Env immunogen available at this time point in an attempt to reach the nAb levels determined as protective in Aim #1. Our data will provide a blueprint towards developing combined-modality vaccines which induce both CTL and nAb responses. We will correlate vitro neutralization in PBMC assays to in vivo protection, thus facilitating optimization of nAb response-based vaccines. As such, our combined-modality approach and its correlation to in vitro neutralization parameters address an important topic in AIDS vaccine development. RELEVANCE (Seeinstructions): Project 2 is designed to answer fundamental questions regarding the protective role of neutralizing anti-HIV- 1 antibodies, the titers required to provide protection against a biologically relevant virus challenge, and the interplay between vaccine-induced CTL and neutralizing antibody-mediated protection.